This invention relates to interference filters, in particular to narrow-band, Fabry-Perot, etalon-type, interference filters, and to the coupling of such filters to the members of a class of telescopes. Each member of this class is characterized by a primary mirror with a perforation therein and by a secondary mirror that partially obstructs light-rays from striking this primary mirror. The secondary mirror redirects those light rays that have fallen upon the primary mirror, and have begun to converge, to a conventional focus, through the aforesaid perforation. The telescope's conventional focus thus is located on the side of the primary mirror farthest from the secondary mirror and is furthermore located close-enough to this rear side that the telescope's drawtube, into which various eyepieces or a CCD and/or a diagonal mirror may be inserted, is not overly long. A narrow-band interference filter is a filter whose half-bandwidth is, in order of magnitude, similar to the shift in the filter's peak, transmitted wavelength that is occasioned by a small departure from normal incidence of a beam of light incident upon and filtered by it. Half-bandwidths of less than 0.6 .ANG. are quite commonly used by amateur observers to watch filaments and flares on the sun's disk, as well as prominences originating in the sun's low chromosphere and expanding into the corona. Most often, although not always, such narrow-band filters are centered on the deep-red wavelength at which the hydrogen atom, at rest with respect to the observer, emits and absorbs light, at 6562.8 .ANG.. Known as H-alpha, or H.alpha., this spectral line appears as a broad absorption feature in the spectrum of the solar disk. The broadening of the line results from random, thermal motions of the absorbing atoms, from turbulent motions of the gases containing them, and in part from collisions with nearby atomic species, a broadening dominated by the Stark effect. By contrast, the luminous, gossamery prominences seen at the sun's limb display the H.alpha.-line in emission only, and the line's width, due nearly entirely to thermal, or Doppler, broadening, is generally much narrower than the broad, disk absorption line. Because the prominences--which appear through a narrow-band filter as bright objects against a black background--re-radiate and scatter light emerging radially outward from the photosphere below them, and because they do this into all solid angles, and further because they are generally cooler than the photosphere, they appear, in H.alpha., to be much fainter than the solar disk. The average, quiescent prominence appears to radiate with an intensity equal to only about 10% the intensity of the residual, H.alpha. emission--known as the "continuum"--of the underlying photosphere. This residual, photospheric, or continuum emission, observed as it escapes from among and between the overlying absorption features of the chromosphere is what gives to the sun's disk in H.alpha. its mottled and gnarled appearance.
The intensity relations just described are depicted in FIG. 32. For the H.alpha. absorption-line profile shown in FIG. 32, see, for example, The Quiet Sun, by Edward G. Gibson, Manned Space-craft Center, NASA, Houston, Tex.; U.S. Government Printing Office, Washington, D.C., 1973, p. 147. For a qualitative discussion of prominence, spectral, line widths, see, for example, Astrophysics of the Sun, by Harold Zirin, Cambridge University Press, New York, 1988, pp. 264-265.
To increase the contrast of the H.alpha.-absorption features observed against the underlying, continuum emission over the sun's disk, an interference filter with the narrowest possible half-bandwidth--admitting the least continuu emission--becomes greatly desirable. It is desirable also, however, to be able to view the entire solar disk plus prominences out to a distance of about 1 solar radius from the sun's limb all at once, the better to be able to survey for, and to follow, interesting activity. This in turn entails viewing an area on the sky with an angular diameter of about 1 degree, the sun, like the moon, subtending an average, angular diameter of about half a degree.
Any telescope that images an extended area such as this necessarily sends converging rays of light to points on an extended image surface, each point of which is actually the focus of a unique cone of light rays. To say that there is a cone, however, implies that there is a certain spread of ray-angles contained within the cone's apex angle, each of which contributes to the illumination of the given point.
This circumstance causes a problem only when a narrow-band, Fabry-Perot, etalon-type interference filter is interposed into the convergent cone, because the filter's peak, transmitted wavelength shifts as a function of the cosine of the angle of incidence of any light-ray falling upon it. For small angles of incidence, this shift is proportional to the square of the angle, expressed in radians (see FIGS. 33 and 34). The focal-ratio of the telescope and the height above the optical axis of the image-point determine the spread of angles within the cone converging toward that point. Each ray within the cone will have, as the ray passes through the interference filter, its own, particular, peak-transmission wavelength, different from the rest-wavelength of interest. Hence, the illumination of each point in the image, made up of many such contributions, together forming the envelope that is the filter's transmission profile at the point, will not be monochromatic. It will contain light coming not only from the center of the spectral line, but it will also contain varying amounts of emission from the neighboring continuum, as well. The faster the imaging system--i.e. the smaller the focal-ratio, or more spread-out the light cones--the more serious this problem becomes.
If the focal ratio is decreased too far, the filter may allow such a large quantity of radiation to enter the detector (the eye, a CCD camera, etc.) from the continuum bordering the absorption line of interest, that all contrast will be lost and few, if any, absorption features will be observed.
Solutions to this image-contrast problem come essentially in two flavors. In the plain-flavor solution, the telescope's focal ratio is increased in one of two ways. Either its aperture D is stopped down, or a negative lens is placed on the optical axis between the telescope's objective and the filter, in order to increase the telescope's effective focal length F.
Aperture reduction is the most-commonly used method among amateur observers, since it requires nothing more than the placing of a sub-diameter stop over the aperture of the telescope--axially for common refractors, or off-axis for such popular catadioptrics as the Meade and Celestron Schmidt-Cassegrains, and the Questar and INTES Maksutov-Cassegrains, among many others. Since a narrow-band, H.alpha. filter must be preceded by a broad-band filter, commonly known as an energy-rejection pre-filter, if it is to be exposed to the sun's energy concentrated in a telescope, such pre-filters are sold in a variety of sub-diameter versions, each sized to obtain a minimum f/30 focal ratio for a given make and aperture of telescope. The pre-filter thus acts as an aperture stop, thereby defining the entrance pupil of the telescope. In the case of catadioptric telescopes, this pre-filter is placed closely adjacent to the refracting element--a meniscus lens in the case of Maksutov-Cassegrains, an afocal correcting plate in the case of Schmidt-Cassegrains--on the side of the element farthest from the primary mirror.
Observation through a narrow-band, Fabry-Perot, etalon-type interference filter does not invariably require an energy-rejection pre-filter, however. Interference filters centered, for example, on the rest wavelength of the Fraunhofer K line, one of the doublet lines of singly ionized Calcium, and the broadest line in the visible, solar spectrum, at 3933.7 .ANG., require no such pre-filter. In these cases, the entrance pupil of the telescope may be reduced by placing an aperture stop as close to the secondary mirror as the construction of telescope will allow and on the side of the secondary mirror farthest from primary mirror.
One serious disadvantage of aperture-reduction as a means for enhancing contrast is that, with increasing magnification of the image, less and less light becomes available per unit solid angle entering the eye--the aperture D being fixed--so that the sun's image actually appears faint. High-power contrast notably deteriorates. At low levels of illumination, the human eye has increasing difficulty resolving even very contrasty detail, as we are reminded whenever we try to read, in a dimly-lit room, black print on a book's white pages. Furthermore, aperture-reduction has no effect on image scale in the focal plane. If the telescope has a long focal length, the entire solar disk may simply be difficult to accommodate within the field-stop of a standard, "low power" eyepiece, not to mention accommodating the surrounding prominences. At 1.13 inches in diameter for every 120 inches of focal length, the sun's disk is, by itself, 1.03 inches across at the conventional focus of a 11" f/10 Schmidt-Cassegrain telescope.
The image-contrast problem may also be addressed by increasing the telescope's focal ratio by means of a negative lens placed in the path of the converging beam, thereby narrowing it and increasing the telescope's effective focal length. This method is occasionally used with slow, f/15 refractors, but it has the disadvantage of increasing the telescope's length, cramping portability. Furthermore, since the aperture D remains fixed, high-power contrast once again deteriorates. The image-scale problem arises again, too, aggravated now by the increase in F.
The spicy-flavor solution to the image-contrast problem involves the use of a so-called telecentric relay, in which light from an imaging objective is first brought to a focus, then is collimated by a lens, next is passed through the narrow-band, Fabry-Perot, etalon-type interference filter, and then is reimaged by a second lens, usually symmetric to the first, optically, whereupon the solar image may be viewed through an eyepiece or reimaged once again through a microscope objective. U.S. Pat. No. 5,125,743 integrates exactly such an optical path, albeit with sophisticated elaborations, into a solar magnetograph. For an in-depth discussion of such telecentric systems, see, for example, The Effect of Telecentric Use of Narrow-Band Filters on Diffraction Limited Imaging by Jacques M. Beckers, National Solar Observatory/NOAO, Tucson, Ariz., and Sunspot, N.M.; paper presented at the SPIE [Society of Photo-optical Instrumentation Engineering] Conference 3355 on Optical Astronomical Instrumentation, Mar. 26-28, 1998.
Commercial, telecentric systems, either integrated into refracting, solar-dedicated telescopes or sold as accessories to the owners of high-end, general-use, refracting telescopes, have been available for some time. These systems, however, all share the contrast deterioration problem at high power outlined above, since both D and F remain fixed, and may share the image-scale problem, if F is large and the relay's lenses are symmetric.
A primary object of the present invention is thus to provide a means for solving the high-power, image-contrast problem, and in fact to provide a means that accommodates a range of image-magnifications, offering at each magnification an image to view that is sufficiently bright as to preserve maximum contrast, and that furthermore solves the whole-disk-plus-surrounding-prominences, image-scale problem, and that accomplishes these useful objectives for a broad class of telescopes, both commercial-grade and professional.